1. Field of the Invention
This invention is generally related to telecommunications systems and apparatuses that employ spreading PN-codes and, in particular, relates to methods and apparatus for hopping PN-codes for mitigation of multi-user interference in code division multiple access (CDMA) systems.
2. Prior Art
Spread spectrum (SS) systems, which may be CDMA systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) PN-code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
There are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF Rc, where fRF represents the center bandpass carrier frequency and Rc represents the PN-code maximum chip rate, which in turn is an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver having its own designated PN-code. Although each receiver receives the entire frequency bandwidth only the signal with the receiver's matching PN-code will appear intelligible, the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
As noted, the DHSS system PN-code sequence spreads the data signal over the available bandwidth such that the carrier appears to be noise-like and random, but is deterministic to a receiver using the same PN-code.
It is well known that the selection of PN-codes in CDMA systems is of critical importance. Ideal PN-codes are perfectly orthogonal in that the autocorrelation function for the PN-code has a large peaked maximum for perfect synchronization of two identical PN-code sequences; and zero cross correlation between different PN-codes sequences.
Of the number of possible sets of orthogonal functions that can be used as PN-code generators, Hadamard functions are recognized as being particularly well suited for their orthogonal properties. Hadamard functions can be described by Hadamard matrices with powers of 2 as ordinary numbers and are well known in the art. Other orthogonal systems can be derived from Hadamard matrices by permuting the columns while still preserving the original orthogonal characteristics.
One such derivation technique is the application of Walsh functions. Walsh functions are a set of binary and orthogonal waveforms that can be used for signal multiplexing purposes, and have long been recognized as having application to telephony. Reference in this regard can be had to an article entitled “The Multiplexing of Telephone Signals by Walsh Functions”, by I. A. Davidson in Applications of Walsh Functions, 1971 proceedings; Second Edition, Eds. R. W. Zeek and A. E. Showalter, pages 177-179.
Another technique for creating PN-codes which are mutually orthogonal is to use a recursive construction technique defined by H. Hubner, “Multiplex Systems Using Sums of Walsh Functions as Carriers”, also in Applications of Walsh Functions, 1971 proceedings, Second Edition, pages 180-191. Reference in this regard can also be had to U.S. Pat. No. 5,751,761, entitled “System and Method for Orthogonal Spread Spectrum Sequence Generation in Variable Data Rate Systems”, by Klein S. Gilhousen.
In modular CDMA systems, such as cell systems where a base station controls what PN-codes are used, there are functions for generating PN-codes that, for all practical purposes, are completely orthogonal. In multiple cell systems, however adjacent cells using a common frequency band may have PN-code sets that may not have low cross-correlation values.
Another source of interference between adjacent cells arises from a common method used to increase the data rate of a DS-CDMA system operating with a fixed PN-code-chipping rate. This method implements variable rate spreading PN-codes where very few chips modulate each symbol in order to increase the effective symbol rate of the system, but at the expense of a decreasing spreading gain. The disadvantage of using the smaller spreading gains is that there will be instances where users in adjacent cells use PN-codes that are highly cross-correlated for a given offset, resulting in potentially significant amounts of interference. In a DS-CDMA system where users are assigned fixed PN-codes, using the wrong PN-code at the wrong offset can result in significant levels of adjacent channel interference for every transmitted symbol. Users in this situation would experience a much lower signal-to-noise ratio than other users in the cell. To combat this worst-case situation, PN cover PN-codes are typically used in the adjacent cells, effectively scrambling the spreading PN-codes and reducing the frequency of perfect and highly correlated levels of interference. A cover PN-code would slightly reduce the signal-to-noise ratio for all users with the aim of preventing any user with a severe degradation in signal-to-noise due to adjacent cell PN-code cross-correlation.
While the use of cover PN-codes appears attractive to solving the problem of worst-case adjacent channel interference resulting from high levels of PN-code cross-correlation, there are several disadvantages with using cover PN-codes.
First, cover PN-codes are difficult in application, as the cover PN-code must be applied to all CDMA channels, including the control and random access channels. This results in potentially longer and more complex acquisition schemes and circuitry.
Another disadvantage that arises when applying a cover PN-code to the matrix of Walsh PN-codes is that the resulting Walsh PN-codes are unbalanced. This means that, over any symbol period the number of +1 valued chips and −1 valued chips are not equal in most of the resulting PN-codes. A balanced PN-code is a desirable property since it implies that the PN-codes are orthogonal to any DC offset in the receiver of the signal. In other words, if the chips are ±1 millivolts in the receiver, but there is a 2 millivolt DC offset in the signal at the input of the despreader, then the despreader would have to multiply the ±1 despreading PN-code with an input signal having values of +3 and +1 millivolts. However, if the PN-code is balanced over a symbol, then the DC offset will not affect the despreading process.
Therefore, it is desirable to provide a method and system to overcome the difficulties with cover PN-codes and to mitigate the worst-case adjacent cell interference problem and provide several advantages not available with cover PN-codes.